It's been discussed elsewhere, but it's worth noting again, that the UK is currently going through a hyper-knife-crime-busting-spree, and as such, right now, they're looking for anyone carrying knives in any way to make examples of.
I would strongly discourage you from carrying any folder longer than your thumbnail in the UK. The calls from policitians and police cheifs for 5-year mandatory prison sentences for carrying knives, etc. are getting a lot of sway, and the police (who have amazing powers in England) will not hesitate to prosecute you.
It's also the case that to qualify for carrying a knife under the 'carrying for a good reason' part of the law pretty much excludes nearly all public places, unless working in a public place is part of your job (i.e. you work for BT, and frequently cut cables in trenches with your SAK or something).
If you are out and about in public and get stopped, they will search you (they can do this), and they will give you a major hard time over any knife yoiu may carrying.
